US11785686B2 - LED drive circuit - Google Patents

LED drive circuit Download PDF

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US11785686B2
US11785686B2 US17/504,565 US202117504565A US11785686B2 US 11785686 B2 US11785686 B2 US 11785686B2 US 202117504565 A US202117504565 A US 202117504565A US 11785686 B2 US11785686 B2 US 11785686B2
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resistor
led
drive circuit
npn transistor
led drive
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US20220039232A1 (en
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Yumin Saigo
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Murata Manufacturing Co Ltd
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    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
    • H05B45/00—Circuit arrangements for operating light-emitting diodes [LED]
    • H05B45/30—Driver circuits
    • H05B45/345—Current stabilisation; Maintaining constant current
    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
    • H05B45/00—Circuit arrangements for operating light-emitting diodes [LED]
    • H05B45/50—Circuit arrangements for operating light-emitting diodes [LED] responsive to malfunctions or undesirable behaviour of LEDs; responsive to LED life; Protective circuits
    • H05B45/56—Circuit arrangements for operating light-emitting diodes [LED] responsive to malfunctions or undesirable behaviour of LEDs; responsive to LED life; Protective circuits involving measures to prevent abnormal temperature of the LEDs
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10H—INORGANIC LIGHT-EMITTING SEMICONDUCTOR DEVICES HAVING POTENTIAL BARRIERS
    • H10H20/00—Individual inorganic light-emitting semiconductor devices having potential barriers, e.g. light-emitting diodes [LED]
    • H10H20/80—Constructional details

Definitions

  • the present invention relates to an LED drive circuit that drives an LED.
  • LEDs Light emitting diodes
  • a battery is often used as a power supply.
  • a constant current circuit may be used to drive an LED.
  • Japanese Patent No. 5079858 discloses a constant current circuit for driving an LED.
  • FIG. 9 illustrates the constant current circuit disclosed in Japanese Patent No. 5079858.
  • a constant current circuit 1000 includes terminals 101 and 102 , NPN transistors 103 and 104 , and resistors 105 and 106 .
  • the terminal 101 is connected to each of a collector of the NPN transistor 103 and one end of the resistor 105 .
  • the other end of the resistor 105 is connected to each of a base of the NPN transistor 103 and a collector of the NPN transistor 104 .
  • An emitter of the NPN transistor 103 is connected to each of one end of the resistor 106 and a base of the NPN transistor 104 , and each of the other end of the resistor 106 and an emitter of the NPN transistor 104 is connected to the terminal 102 .
  • a base-emitter threshold voltage of the NPN transistors 103 and 104 which is the threshold voltage at which the NPN transistors 103 and 104 change from an off state to an on state, is 0.6 V. That is, when the voltage applied to the bases of the NPN transistors 103 and 104 is less than 0.6 V, the NPN transistors 103 and 104 are turned off, and when the voltage applied to the bases of the NPN transistors 103 and 104 is 0.6 V or more, the NPN transistors 103 and 104 are turned on.
  • the resistor 106 has a resistance value of 2 ⁇ .
  • the LED to be driven is connected to a point X between the terminal 101 and the collector of the NPN transistor 103 .
  • the terminal 101 When the terminal 101 is connected to a DC positive power supply and the terminal 102 is connected to a ground, the voltage of 0.6 V or more is applied to the base of the NPN transistor 103 via the resistor 105 , and the NPN transistor 103 is turned on. As a result, current flows through the LED connected to the point X, and the LED is driven. As a result, the LED emits light. In addition, the current also flows through the resistor 106 .
  • the base-emitter threshold voltage of the NPN transistor 104 is 0.6 V and the resistance value of the resistor 106 is 2 ⁇
  • a current of about 300 mA always flows through the resistor 106 .
  • the current of about 300 mA also always flows through the LED connected to the point X, limited to the current value of the current flowing through the resistor 106 .
  • the voltage applied to the point Y is less than 0.6 V, and the NPN transistor 104 is turned off.
  • the current flowing through the resistor 106 is 300 mA or more, the voltage applied to the point Y is 0.6 V or more, and the NPN transistor 104 is turned on.
  • the NPN transistor 103 is turned on, and the current flows through the LED and the resistor 106 . Then, when the current flowing through the resistor 106 becomes 300 mA or more, the voltage applied to the point Y becomes 0.6 V or more, and the NPN transistor 104 is turned on. Then, when the NPN transistor 104 is turned on, the voltage applied to the base of the NPN transistor 103 becomes less than 0.6 V, and the NPN transistor 103 is turned off.
  • the NPN transistor 103 when the NPN transistor 103 is turned off, the current does not flow through the LED and the resistor 106 , the voltage applied to the point Y becomes less than 0.6 V, and the NPN transistor 104 is turned off. Then, when the NPN transistor 104 is turned off, the voltage of 0.6 V or more is applied to the base of the NPN transistor 103 , and the NPN transistor 103 is turned on again. Then, the current flows through the LED and the resistor 106 again.
  • the base-emitter threshold voltage of the NPN transistor 104 which is the threshold voltage at which the NPN transistor 104 changes from the off state to the on state, has a temperature characteristic, and the threshold voltage fluctuates depending on the ambient temperature around the NPN transistor 104 . More specifically, when the ambient temperature around the NPN transistor 104 increases, the base-emitter threshold voltage of the NPN transistor 104 decreases, and when the ambient temperature around the NPN transistor 104 decreases, the base-emitter threshold voltage of the NPN transistor 104 increases. For example, in the certain NPN transistor 104 , every time the ambient temperature around the NPN transistor 104 increases by 1° C., the base-emitter threshold voltage of the NPN transistor 104 decreases by 0.002 V.
  • the decrease in the base-emitter threshold voltage of the NPN transistor 104 due to the increase in the ambient temperature around the NPN transistor 104 means that the current value of the current flowing through the LED becomes smaller than the design value due to the increase in the ambient temperature around the NPN transistor 104 . Therefore, the constant current circuit 1000 has a problem that the current value of the current flowing through the LED decreases due to the increase in the ambient temperature around the NPN transistor 104 , and the luminance of the LED decreases.
  • the constant current circuit 1000 has a problem that there is no mechanism for limiting the current value of the current flowing through the LED if the temperature of the LED becomes abnormally high. That is, in the constant current circuit 1000 , if the temperature of the LED exceeds the junction temperature, the current having the high current value continues to flow through the LED, and thus there is a problem that the LED fails or the product life of the LED is shortened.
  • Preferred embodiments of the present invention provide LED drive circuits in each of which, if ambient temperature around a LED drive circuit fluctuates, current having a constant current value stably flows through an LED, so that the LED always emits light at a constant luminance.
  • a preferred embodiment of the present invention provides an LED drive circuit including a power supply terminal, an LED, a first switch, a second switch, a first resistor, a second resistor, and a third resistor.
  • the power supply terminal, the LED, the first switch, and the first resistor are connected in series.
  • the power supply terminal, the third resistor, and the second switch are connected in series.
  • a connection point between the third resistor and the second switch, and a control terminal of the first switch are connected.
  • the second resistor is connected between a connection point between the first switch and the first resistor, and a control terminal of the second switch.
  • a first PTC thermistor, or a second PTC thermistor and an NTC thermistor connected in series with each other are connected to a connection point between the control terminal of the second switch and the second resistor.
  • each of the LED drive circuits if ambient temperature around the LED drive circuit fluctuates, current having a constant current value stably flows through the LED, so that the LED always emits light at constant luminance.
  • each of the LED drive circuits according to preferred embodiments of the present invention can limit a current value of the current flowing through the LED when a temperature of the LED becomes abnormally high, it is possible to prevent failure of the LED due to continuous flow of the current having a high current value through the LED, and it is possible to prolong the useful life of the LED.
  • FIG. 1 is an equivalent circuit diagram of an LED drive circuit according to a first preferred embodiment of the present invention.
  • FIG. 2 is a graph illustrating a resistance temperature characteristic of a first PTC thermistor of the LED drive circuit according to the first preferred embodiment of the present invention.
  • FIG. 3 is a graph illustrating a relationship between ambient temperature and voltage V P at a point P in the LED drive circuit according to the first preferred embodiment of the present invention.
  • FIG. 4 is an equivalent circuit diagram of an LED drive circuit according to a second preferred embodiment of the present invention.
  • FIG. 5 is a graph illustrating a resistance temperature characteristic of a second PTC thermistor and an NTC thermistor connected in series in the LED drive circuit according to the second preferred embodiment of the present invention.
  • FIG. 6 is an equivalent circuit diagram of an LED drive circuit according to a third preferred embodiment of the present invention.
  • FIG. 7 is an equivalent circuit diagram of an LED drive circuit according to a fourth preferred embodiment of the present invention.
  • FIG. 8 is an equivalent circuit diagram of an LED drive circuit according to a fifth preferred embodiment of the present invention.
  • FIG. 9 is an equivalent circuit diagram of a constant current circuit disclosed in Japanese Patent No. 5079858.
  • FIG. 1 is an equivalent circuit diagram of an LED drive circuit according to a first preferred embodiment of the present invention.
  • an LED drive circuit 100 includes a power supply terminal 1 , a switch 2 , an LED 3 , a first NPN transistor 4 as a first switching element, a second NPN transistor 5 as a second switching element, a first resistor 6 , a second resistor 7 , a third resistor 8 , a resistor 9 , and a first PTC thermistor 10 .
  • the power supply terminal 1 is connected to a power supply 60 .
  • the power supply 60 is a DC positive power supply.
  • the power supply 60 is, for example, a battery mounted on an automobile.
  • the power supply 60 uses, for example, 12 V as a reference voltage, but the voltage value fluctuates in a range of, for example, about 9 V to about 16 V depending on various conditions.
  • a base-emitter threshold voltage of the first NPN transistor 4 and the second NPN transistor 5 which is the threshold voltage at which the first NPN transistor 4 and the second NPN transistor 5 change from an off state to an on state, is, for example, about 0.68 V at about 25° C.
  • the base-emitter threshold voltage of the first NPN transistor 4 and the second NPN transistor 5 has a temperature characteristic, and every time ambient temperature around the LED drive circuit 100 , that is, ambient temperature around the first NPN transistor 4 and the second NPN transistor 5 increases by about 1° C., the base-emitter threshold voltage decreases by about 0.002 V.
  • the first resistor 6 has a resistance value of about 4.7 ⁇ .
  • the second resistor 7 has a resistance value of about 1 ⁇ .
  • resistance values of the third resistor 8 and the resistor 9 can be appropriately set on condition that the first NPN transistor 4 is turned on when the switch 2 is turned on. Note that the resistor 9 can also be omitted.
  • the first PTC thermistor 10 has a resistance value of about 470 ⁇ at about 25° C.
  • the first PTC thermistor 10 is, for example, a ceramic PTC thermistor.
  • the first PTC thermistor 10 includes a resistance temperature characteristic.
  • FIG. 2 is a graph illustrating the resistance temperature characteristic of the first PTC thermistor of the LED drive circuit according to the first preferred embodiment of the present invention. As illustrated in FIG. 2 , the first PTC thermistor 10 has a temperature range showing a clear positive resistance temperature characteristic at a temperature higher than the Curie temperature. On the other hand, the first PTC thermistor 10 has a temperature region showing a negative resistance temperature characteristic at a temperature lower than the Curie temperature.
  • the first PTC thermistor of the present preferred embodiment exhibits the negative resistance temperature characteristic at about 25° C. or more and about 90° C. or less, and exhibits the positive resistance temperature characteristic at about 125° C. or more.
  • the resistance value of the first PTC thermistor 10 of the present preferred embodiment is about twice as large as the resistance value thereof at about 25° C., which is room temperature, at about 135° C., and is about 10 times as large as the resistance value thereof at 25° C. at about 145° C.
  • the ceramic PTC thermistor is used as the first PTC thermistor 10 .
  • the first PTC thermistor 10 is disposed in the vicinity of the second NPN transistor 5 . Therefore, ambient temperature around the first PTC thermistor 10 is the same or substantially the same as the ambient temperature around the second NPN transistor 5 . In addition, the ambient temperature around the second NPN transistor 5 and the first PTC thermistor 10 is affected by heat generated by light emission of the LED 3 , and when the temperature of the LED 3 abnormally increases, the ambient temperature around the second NPN transistor 5 and the first PTC thermistor 10 also rapidly increases.
  • a connection relationship among the respective components of the LED drive circuit 100 is as follows.
  • the power supply terminal 1 is connected to one end of the switch 2 .
  • the other end of the switch 2 is connected to each of an anode of the LED 3 and one end of the third resistor 8 .
  • a cathode of the LED 3 is connected to a collector of the first NPN transistor 4 .
  • An emitter of the first NPN transistor 4 is connected to each of one end of the first resistor 6 and one end of the second resistor 7 .
  • a connection point between the emitter of the first NPN transistor 4 , the one end of the first resistor 6 , and the one end of the second resistor 7 is a point P.
  • the other end of the first resistor 6 is connected to a ground.
  • the other end of the third resistor 8 is connected to each of one end of the resistor 9 and a collector of the second NPN transistor 5 .
  • An emitter of the second NPN transistor 5 is connected to the ground.
  • the other end of the resistor 9 is connected to a base of the first NPN transistor 4 .
  • the base of the first NPN transistor 4 is a control terminal.
  • the other end of the second resistor 7 is connected to each of a base of the second NPN transistor 5 and one end of the first PTC thermistor 10 .
  • a connection point between the other end of the second resistor 7 , the base of the second NPN transistor 5 , and the one end of the first PTC thermistor 10 is a point Q.
  • the base of the second NPN transistor 5 is a control terminal.
  • the other end of the first PTC thermistor 10 is connected to the ground.
  • the LED drive circuit 100 performs the following operation when the switch 2 is turned on. However, unless otherwise specified, the ambient temperature around the second NPN transistor 5 is about 25° C., which is the room temperature.
  • the base-emitter threshold voltage of the second NPN transistor 5 which is the threshold voltage at which the second NPN transistor 5 changes from the off state to the on state, is about 0.68 V at about 25° C.
  • voltage V Q at the point Q is about 0.68 V, which is the base-emitter threshold voltage of the second NPN transistor 5
  • voltage V P at the point P is about 1.39 V.
  • a current value of the current flowing from the point P to the ground is about 298 mA.
  • a current value of the current flowing through the LED 3 is also about 298 mA.
  • the switch 2 when the switch 2 is turned on, the current flows through the LED 3 and the first resistor 6 , and the LED 3 emits light.
  • the current value of the current flowing through the LED 3 and the current value of the current flowing from the point P to the ground become about 298 mA or more, the voltage V Q at the point Q becomes about 0.68 V or more, and the second NPN transistor 5 is turned on.
  • the voltage applied to the base of the first NPN transistor 4 becomes less than about 0.68 V, and the first NPN transistor 4 is turned off.
  • the first NPN transistor 4 when the first NPN transistor 4 is turned off, the current does not flow through the LED 3 and the first resistor 6 , the voltage V Q at the point Q becomes less than about 0.68 V, and the second NPN transistor 5 is turned off.
  • the second NPN transistor 5 when the second NPN transistor 5 is turned off, the voltage of about 0.68 V or more is applied to the base of the first NPN transistor 4 , and the first NPN transistor 4 is turned on again. Then, the current flows through the LED 3 and the first resistor 6 again.
  • the above operation is repeated at high speed. Therefore, if the voltage value of the power supply 60 fluctuates, the current of about 298 mA always stably flows through the LED 3 , and the LED stably emits light at a constant luminance.
  • this state may be referred to as a state in which the voltage V Q at the point Q is fixed at about 0.68 V.
  • this state may be referred to as a state in which the voltage V P at the point P is fixed at about 1.39 V.
  • the above is the operation of the LED drive circuit 100 when the ambient temperature around the second NPN transistor 5 is maintained at about 25° C., which is the room temperature.
  • the base-emitter threshold voltage of the second NPN transistor 5 which is the threshold voltage at which the second NPN transistor 5 changes from the off state to the on state, has the temperature characteristic, and every time the ambient temperature increases by about 1° C., the base-emitter threshold voltage decreases by about 0.002 V.
  • the voltage V Q at the point Q becomes smaller than about 0.68 V and the voltage V P at the point P becomes smaller than about 1.39 V. Then, as a result, the current value of the current flowing through the LED 3 becomes smaller than about 298 mA, and the luminance of the LED 3 becomes lower than the design value.
  • the LED drive circuit 100 uses the resistance temperature characteristic of the first PTC thermistor 10 to correct the decrease in the current value of the current flowing through the LED 3 due to the decrease in the base-emitter threshold voltage of the second NPN transistor 5 due to the increase in the ambient temperature. That is, the LED drive circuit 100 includes a temperature compensation function.
  • the first PTC thermistor 10 exhibits the negative resistance temperature characteristic at about 25° C. or more and about 90° C. or less, and the resistance value decreases as the ambient temperature increases. That is, when the temperature of the first PTC thermistor 10 increases from about 25° C., the resistance value of about 470 ⁇ at about 25° C. decreases (see FIG. 2 ). Then, due to the decrease in the resistance value of the first PTC thermistor 10 due to the increase in the ambient temperature, the current value of the current flowing from the point Q to the ground increases, the current value of the current flowing from the point P to the ground increases, and resultantly the current value of the current flowing through the LED 3 increases.
  • the decrease in the current value of the current flowing through the LED 3 due to the decrease in the base-emitter threshold voltage of the second NPN transistor 5 due to the increase in the ambient temperature is canceled by the increase in the current value of the current flowing through the LED 3 due to the decrease in the resistance value of the first PTC thermistor 10 due to the increase in the ambient temperature.
  • the current value of the current flowing through the LED 3 does not decrease from about 298 mA.
  • the luminance of the LED 3 does not decrease. Therefore, the LED 3 stably emits light at a constant luminance.
  • FIG. 3 is a graph illustrating a relationship between the ambient temperature and the voltage V P at the point P in the LED drive circuit according to the first preferred embodiment of the present invention.
  • a relationship between the ambient temperature around the second NPN transistor 5 and the first PTC thermistor 10 and the voltage V P at the point P in the LED drive circuit 100 is indicated by a solid line.
  • the voltage V P at the point P hardly fluctuates and is flat or substantially flat.
  • the LED drive circuit 100 when the ambient temperature around the second NPN transistor 5 and the first PTC thermistor 10 exceeds about 120° C., the voltage V P at the point P rapidly decreases. This is because the first PTC thermistor 10 starts to exhibit the positive resistance temperature characteristic. Then, when the voltage V P at the point P decreases, the current value of the current flowing through the LED 3 decreases.
  • the LED drive circuit 100 prevents the temperature of the LED 3 from further increasing when the temperature of the LED 3 becomes abnormally high, and prevents the temperature of the LED 3 from reaching the junction temperature, for example, about 125° C. to about 150° C. That is, in the LED drive circuit 100 , when the temperature of the LED 3 becomes abnormally high, for example, exceeding about 120° C., the ambient temperature around the second NPN transistor 5 and the first PTC thermistor 10 greatly increases, so that the voltage V P at the point P greatly decreases, and the current value of the current flowing through the LED 3 greatly decreases. Therefore, further temperature increase of the LED 3 is prevented, and the LED 3 is protected.
  • the above-described LED drive circuit 100 (see FIG. 1 ) was manufactured as an example.
  • the first resistor 6 has the resistance value of about 4.7 ⁇ .
  • the second resistor 7 has the resistance value of about 1 ⁇ .
  • the first PTC thermistor 10 has the resistance value of about 470 ⁇ at about 25° C.
  • an LED drive circuit according to a comparative example was manufactured.
  • the LED drive circuit according to the comparative example has a configuration in which the second resistor 7 and the first PTC thermistor 10 are omitted from the LED drive circuit 100 , and the emitter of the first NPN transistor 4 is directly connected to the base of the second NPN transistor 5 .
  • the first resistor 6 had a resistance value of about 2.2 ⁇ . The resistance value of the first resistor 6 is shown in Table 1.
  • the ambient temperature around the second NPN transistor 5 and the first PTC thermistor 10 was increased from about 22° C. to about 140° C., and the voltage V P at the point P was measured.
  • the ambient temperature around the second NPN transistor 5 was increased from about 22° C. to about 140° C., and the voltage V P at the point P was measured.
  • FIG. 3 a relationship between the ambient temperature around the second NPN transistor 5 and the voltage V P at the point P in the LED drive circuit according to the comparative example is indicated by a broken line.
  • the voltage V P at the point P linearly decreases as the ambient temperature around the second NPN transistor 5 increases.
  • the current value of the current flowing through the LED 3 was measured when the ambient temperature around the second NPN transistor 5 and the first PTC thermistor 10 was about 25° C., about 90° C., and about 135° C.
  • the current value of the current flowing through the LED 3 was measured when the ambient temperature around the second NPN transistor 5 was about 25° C., about 90° C., and about 135° C. The respective measurement results are shown in Table 1.
  • the current value of the current flowing through the LED 3 of the example was about 298 mA when the ambient temperature was about 25° C., and about 283 mA when the ambient temperature was about 90° C.
  • the current value of the current flowing through the LED 3 decreased by only about 5%. This is considered to be an advantageous effect caused by the negative resistance temperature characteristic of the first PTC thermistor 10 canceling the decrease in the current value of the current flowing through the LED 3 due to the decrease in the base-emitter threshold voltage of the second NPN transistor 5 due to the increase in the ambient temperature.
  • the current value of the current flowing through the LED 3 of the comparative example was about 309 mA when the ambient temperature was about 25° C., and about 250 mA when the ambient temperature was about 90° C. As the ambient temperature increased from about 25° C. to about 90° C., the current value of the current flowing through the LED 3 decreased by about 20%.
  • the current value of the current flowing through the LED 3 was about 150 mA, and the current flowing through the LED 3 was reduced or prevented. Therefore, in the example, when the temperature of the LED 3 becomes abnormally high exceeding about 120° C., the current value of the current flowing through the LED 3 is reduced or prevented, and further temperature increase of the LED 3 can be reduced or prevented. On the other hand, in the comparative example, if the ambient temperature reached about 135° C., the current value of the current flowing through the LED 3 was about 209 mA, and the current flowing through the LED 3 was not reduced or prevented.
  • the temperature of the LED 3 may continue to increase, and the temperature of the LED 3 may reach the junction temperature.
  • FIG. 4 is an equivalent circuit diagram of an LED drive circuit according to a second preferred embodiment of the present invention.
  • an LED drive circuit 200 is modified in a portion of the configuration of the LED drive circuit 100 according to the first preferred embodiment described above.
  • the first PTC thermistor 10 is connected between the point Q, which is the connection point between the other end of the second resistor 7 and the base of the second NPN transistor 5 , and the ground
  • a second PTC thermistor 20 and an NTC thermistor 21 connected in series with each other are connected between the point Q, which is the connection point between the other end of the second resistor 7 and the base of the second NPN transistor 5 , and the ground.
  • Other configurations of the LED drive circuit 200 are the same or substantially the same as those of the LED drive circuit 100 .
  • FIG. 5 is a graph illustrating a resistance temperature characteristic of the second PTC thermistor and the NTC thermistor connected in series in the LED drive circuit according to the second preferred embodiment of the present invention.
  • a combined resistance temperature characteristic of the second PTC thermistor 20 and the NTC thermistor 21 connected in series is indicated by a solid line.
  • the resistance temperature characteristic of the first PTC thermistor 10 of the LED drive circuit 100 according to the first preferred embodiment is indicated by a broken line.
  • the slope of the negative resistance temperature characteristic appearing in the temperature region lower than the Curie temperature is larger than that of the resistance temperature characteristic of the first PTC thermistor 10 of the LED drive circuit 100 . Therefore, the second PTC thermistor 20 and the NTC thermistor 21 of the LED drive circuit 200 can further increase the current value of the current flowing through the LED 3 when the ambient temperature increases, as compared with the first PTC thermistor 10 of the LED drive circuit 100 .
  • the LED drive circuit 200 is useful when the range for correcting the decrease in the current value of the current flowing through the LED 3 due to the decrease in the threshold voltage of the second NPN transistor 5 due to the increase in the ambient temperature is large.
  • FIG. 6 is an equivalent circuit diagram of an LED drive circuit according to a third preferred embodiment of the present invention.
  • an LED drive circuit 300 is also modified in a portion of the configuration of the LED drive circuit 100 according to the first preferred embodiment described above.
  • the first NPN transistor 4 is the first switching element and the second NPN transistor 5 is the second switching element
  • a first PNP transistor 34 is the first switching element and a second PNP transistor 35 is the second switching element.
  • Other configurations of the LED drive circuit 300 are the same or substantially the same as those of the LED drive circuit 100 .
  • the cathode of the LED 3 is connected to an emitter of the first PNP transistor 34 .
  • a collector of the first PNP transistor 34 is connected to each of the one end of the first resistor 6 and the one end of the second resistor 7 .
  • a base of the first PNP transistor 34 is a control terminal.
  • the other end of the third resistor 8 is connected to each of the one end of the resistor 9 and an emitter of the second PNP transistor 35 .
  • a collector of the second PNP transistor 35 is connected to the ground.
  • a base of the second PNP transistor 35 is a control terminal.
  • the decrease in the current value of the current flowing through the LED 3 due to the decrease in the base-emitter threshold voltage of the second PNP transistor 35 due to the increase in the ambient temperature is canceled by the increase in the current value of the current flowing through the LED 3 due to the decrease in the resistance value of the first PTC thermistor 10 due to the increase in the ambient temperature. Therefore, if the ambient temperature increases, the current value of the current flowing through the LED 3 does not decrease, the luminance of the LED 3 does not decrease, and the LED 3 stably emits light at constant luminance.
  • the resistance temperature characteristic of the first PTC thermistor 10 is used to correct the decrease in the current value of the current flowing through the LED 3 due to the decrease in the base-emitter threshold voltage of the second PNP transistor 35 due to the increase in the ambient temperature. That is, the LED drive circuit 300 includes a temperature compensation function.
  • FIG. 7 is an equivalent circuit diagram of an LED drive circuit according to a fourth preferred embodiment of the present invention.
  • an LED drive circuit 400 according to the fourth preferred embodiment is also modified in a portion of the configuration of the LED drive circuit 100 according to the first preferred embodiment described above.
  • the first NPN transistor 4 is the first switching element and the second NPN transistor 5 is the second switching element
  • a first N-channel FET 44 is the first switching element
  • a second N-channel FET 45 is the second switching element.
  • Other configurations of the LED drive circuit 400 are the same or substantially the same as those of the LED drive circuit 100 .
  • the cathode of the LED 3 is connected to a drain of the first N-channel FET 44 .
  • a source of the first N-channel FET 44 is connected to each of the one end of the first resistor 6 and the one end of the second resistor 7 .
  • the other end of the third resistor 8 is connected to each of the one end of the resistor 9 and a drain of the second N-channel FET 45 .
  • the other end of the resistor 9 is connected to a gate of the first N-channel FET 44 .
  • the gate of the first N-channel FET 44 is a control terminal.
  • the other end of the second resistor 7 is connected to each of a gate of the second N-channel FET 45 and the one end of the first PTC thermistor 10 .
  • the gate of the second N-channel FET 45 is a control terminal.
  • a source of the second N-channel FET 45 is connected to the ground.
  • the decrease in the current value of the current flowing through the LED 3 due to the decrease in gate-source threshold voltage of the second N-channel FET 45 due to the increase in the ambient temperature is canceled by the increase in the current value of the current flowing through the LED 3 due to the decrease in the resistance value of the first PTC thermistor 10 due to the increase in the ambient temperature. Therefore, if the ambient temperature increases, the current value of the current flowing through the LED 3 does not decrease, the luminance of the LED 3 does not decrease, and the LED 3 stably emits light at a constant luminance.
  • the resistance temperature characteristic of the first PTC thermistor 10 is used to correct the decrease in the current value of the current flowing through the LED 3 due to the decrease in the gate-source threshold voltage of the second N-channel FET 45 due to the increase in the ambient temperature. That is, the LED drive circuit 400 includes a temperature compensation function.
  • FIG. 8 is an equivalent circuit diagram of an LED drive circuit 500 according to a fifth preferred embodiment of the present invention.
  • the LED drive circuit 500 is also modified in a portion of the configuration of the LED drive circuit 100 according to the first preferred embodiment described above. Specifically, in the LED drive circuit 100 , the first NPN transistor 4 is the first switching element and the second NPN transistor 5 is the second switching element, whereas in the LED drive circuit 500 , a first P-channel FET 54 is the first switching element and a second P-channel FET 55 is the second switching element. Other configurations of the LED drive circuit 500 are the same or substantially the same as those of the LED drive circuit 100 .
  • the cathode of the LED 3 is connected to a source of the first P-channel FET 54 .
  • a drain of the first P-channel FET 54 is connected to each of the one end of the first resistor 6 and the one end of the second resistor 7 .
  • the other end of the third resistor 8 is connected to each of the one end of the resistor 9 and a source of the second P-channel FET 55 .
  • the other end of the resistor 9 is connected to a gate of the first P-channel FET 54 .
  • the gate of the first P-channel FET 54 is a control terminal.
  • the other end of the second resistor 7 is connected to each of a gate of the second P-channel FET 55 and the one end of the first PTC thermistor 10 .
  • the gate of the second P-channel FET 55 is a control terminal.
  • a drain of the second P-channel FET 55 is connected to the ground.
  • the decrease in the current value of the current flowing through the LED 3 due to the decrease in gate-source threshold voltage of the second P-channel FET 55 due to the increase in the ambient temperature is canceled by the increase in the current value of the current flowing through the LED 3 due to the decrease in the resistance value of the first PTC thermistor 10 due to the increase in the ambient temperature. Therefore, if the ambient temperature increases, the current value of the current flowing through the LED 3 does not decrease, the luminance of the LED 3 does not decrease, and the LED 3 stably emits light at constant luminance.
  • the resistance temperature characteristic of the first PTC thermistor 10 is used to correct the decrease in the current value of the current flowing through the LED 3 due to the decrease in the gate-source threshold voltage of the second P-channel FET 55 due to the increase in the ambient temperature. That is, the LED drive circuit 500 includes a temperature compensation function.
  • the LED drive circuits 100 , 200 , 300 , 400 , and 500 according to the first to fifth preferred embodiments have been described above.
  • the present invention is not limited to the contents described above, and various modifications can be made.
  • the same type of semiconductor elements for example, the NPN transistor and the NPN transistor
  • different types of semiconductor elements for example, the NPN transistor and the PNP transistor
  • the first switching element includes the first NPN transistor including the NPN transistor, the first PNP transistor including the PNP transistor, the first N-channel FET including the N-channel FET, the first P-channel FET including the P-channel FET, or the like, for example.
  • the second switching element includes the second NPN transistor including the NPN transistor, the second PNP transistor including the PNP transistor, the second N-channel FET including the N-channel FET, and the second P-channel FET including the P-channel FET, or the like, for example.
  • the first PTC thermistor, or the second PTC thermistor and the NTC thermistor connected in series with each other exhibit the negative resistance temperature characteristic at a temperature of about 25° C. or more and about 90° C. or less, and exhibit the positive resistance temperature characteristic at a temperature of about 125° C. or more.
  • the current value of the current flowing through the LED can be satisfactorily limited.
  • the first PTC thermistor or the second PTC thermistor is a ceramic PTC thermistor. In this case, it is possible to satisfactorily correct the decrease in the current value of the current flowing through the LED due to the decrease in the threshold voltage of the second switching element due to the increase in the ambient temperature around the second switching element.
  • the power supply connected to the power supply terminal is a battery.
  • the voltage of the battery is likely to fluctuate, but also in this case, the current having a constant current value flows through the LED, and the luminance of the LED can be maintained constant.

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JP7184183B2 (ja) 2022-12-06
CN113924661A (zh) 2022-01-11
DE112020002722T5 (de) 2022-02-17
CN113924661B (zh) 2024-02-13
US20220039232A1 (en) 2022-02-03
JPWO2020241247A1 (https=) 2020-12-03
WO2020241247A1 (ja) 2020-12-03

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